1. Technical Field
This invention relates to an image processing apparatus for performing image processing for an image pickup signal output from an image pickup device, an endoscope and a computer readable medium storing an image processing program.
2. Description of the Related Art
An endoscope using a CCD or CMOS image sensor as an image pickup device is already often used in the medical field. The endoscopes are roughly classified into (i) a frame sequential image pickup method of using an image pickup device capable of picking up a monochrome image and switching a filter for transmitting light of wavelength ranges of R (red), G (green), B (blue), and IR (infrared) in front of a light source for illuminating a subject through a fiber in synchronization with the field frequency of the image pickup device (for example, see Japanese Patent No. 2648494) and (ii) a simultaneous image pickup method of picking up an image using a single-plate image pickup device provided with a color filter for transmitting light of wavelength ranges of R, G, and B and an illumination light source that emits white light.
The frame sequential image pickup method is a method of rotating a plurality of filters having different spectral transmissivitys in front of a light source and picking up a plurality of images illuminated by light of different wavelengths and then combining the images into a color image. Thus, for example, if RGB transmission filters are used as the filters switched in front of the light source, color image data having three color information pieces of RGB in one pixel data can be provided. Also, if IR filters of two wavelengths put into a narrow band easily absorbed by hemoglobin in blood are switched in sequence, infrared image data in which one pixel data only has information of an infrared region can be provided. An image based on the color image data enables a user to visually check an appearance of a part to be tested, and an image based on the infrared image data enables the user to visually check information, etc., of blood capillary of mucosal surface layer and a mucosal minute pattern in the part to be tested. However, in the frame sequential image pickup, a color shift occurs for a subject involving motion, resulting in image interference.
On the other hand, the simultaneous image pickup method is a method of obtaining color image data by picking up an image and then performing image processing for the color image data, thereby generating infrared image data. According to this method, a color shift does not occur for a subject involving motion, but there is a problem of low information accuracy of the infrared image data.
Thus, an image pickup device used with an endoscope can obtain color image data in which one pixel has three-color (RGB) information and infrared image data in which one pixel only has infrared information by the apparatus configuration disclosed in Japanese Patent No. 2648494 or the image processing. Particularly, the infrared image data is used to check information, etc., of blood capillary of mucosal surface layer and a mucosal minute pattern in the part to be tested. Therefore, high contrast is demanded for the infrared image data.
Also, color reproducibility of the RGB color image data will be described. Usually, R, G, and B color filters also transmit infrared-region wavelength. Thus, if light passing through the R, G, and B color filters is detected in the photoelectric conversion elements and RGB color image data is generated, the color reproducibility thereof is not good. Then, an infrared (IR) cut filter is provided in front of an image pickup device using R, G, and B color filters so that the light passing through the R, G, and B color filters does not contain infrared-region light. Thereby, the color reproducibility is improved.
However, an IR cut filter having a steep IR cut property is expensive and is at high cost. An IR cut filter must be provided in front of the image pickup device and miniaturization of whole system using the image pickup device is also hindered. If an attempt is made to use an IR cut filter in the apparatus described in Japanese Patent No. 2648494, only when R, G, and B color filters come in front of the image pickup device, the IR cut filter needs to be placed in front of the R, G or B filter, and the mechanism and control of the system become complicated.